Field of the Invention
The present invention relates to image capturing apparatuses and control methods therefor, and particularly relates to image capturing apparatuses that perform focus adjustment based on an image signal obtained from an image sensor, and to control methods therefor.
Description of the Related Art
Most digital cameras now have display units such as LCDs or the like, and it has become typical to shoot an image while observing an object displayed in the display unit; as such, these display units are used when preparing to shoot still images and when shooting moving pictures. Furthermore, digital cameras that carry out focusing operations continuously during display are under development, and it is necessary to display an image of an object in the display unit inconspicuously even when such focusing operations are being carried out. Compact digital cameras in particular often use a contrast detection-based focus adjustment system, but doing so poses the following problems.
In a contrast detection-based system, the position of a focus lens in an optical system when the contrast of an image obtained by an image sensor forms a local maximum is found while moving the focus lens in the direction of the optical axis, and is taken as an in-focus position. The in-focus position is typically evaluated based on a shape created by AF evaluation values calculated by extracting a high-frequency component of the image obtained by the image sensor on a frame-by-frame basis, and the same image is displayed in a liquid-crystal display unit and recorded as moving pictures. Accordingly, it has been necessary to generate the image data taking into consideration both the display/moving picture recording and focusing operations in the case where the focusing operations are carried out while displaying an object in the liquid-crystal display unit and carrying out operations for recording moving pictures as described above. As such, the framerate will drop when attempting to generate high-resolution image data according to the requirements for display/moving picture recording, which is disadvantageous in focusing operations that carry out control for reducing the period for which the image is out of focus to the greatest extent possible.
The specification of Japanese Patent No. 4558830 discloses the following digital camera, in order to solve such a problem. An image sensor capable of a process for thinning an image signal of an object and a thinning control unit that controls readout frames and a thinning rate are provided, and control is carried out so that image data is read out at a higher readout framerate during a focusing operation period than during a normal operation period. Furthermore, in the frames during the focusing operation period and immediately before the focusing operation period, control is carried out for reading out image data from the image sensor at a higher thinning rate than that used in the normal operation period. Doing so makes it possible to operate at a framerate suited to focusing operations during the focusing operation period while also maintaining display quality during the normal operation period, and measures are taken to ensure a smooth transition between the two.
Meanwhile, the specification of Japanese Patent No. 4797522 discloses the following digital camera. A light path splitter that splits the optical path of light from an object into two and two image sensors that convert the respective beams of object light obtained from the splitting into image data are provided; one of the image sensors generates a display image, and the other is partially used to handle focusing operations. This makes it possible to carry out focusing operations while maintaining display quality in the normal operation period and the focusing operation period.
However, according to the technique disclosed in the specification of Japanese Patent No. 4558830, the thinning rate in the normal operation period is different from the thinning rate in the focusing operation period, and thus the quality of the image displayed in the liquid-crystal display unit will invariably drop during the focusing operation period. Likewise, there is a problem in that in the case where a moving picture is recorded using this technique, a difference in the quality of the recorded image will remain between the normal operation period and the focusing operation period.
On the other hand, according to the technique disclosed in the specification of Japanese Patent No. 4797522, the display of the object during the normal operation period and the focusing operation during the focusing operation period are realized in respectively optimal forms. However, it is necessary to provide an optical element for branching the optical path, the two image sensors, a driving unit for driving the respective image sensors, and so on, and there is thus a problem in that it is difficult to apply this technique in compact digital cameras, where low costs and downsizing is demanded.